300W actual or apparent power? If 300W actual power, well PSU are rated in apparent power. Depending on their efficiency, they may get nowhere near their rated power. I think about 80% efficiency is about typical under high loads, so that 500W power supply is now down to 400W. (cheaper power supplies have worse efficiencies as well, and I think generally the efficiencies under low utilizations are worse...age also effects efficiency)
~300W from the wall; It's not hard to find these reviews. Which is why the 50% bigger PSU I specced for the boss isn't going to puke and die.
Further more, better quality PSU's will also have Active Power Factor Correction, which further increases the efficiency at higher load. And if you want to argue about how PSU's that are specced within 80% of the machine's current draw are going to die, then tell it to all the OEM's out there who do the same (and have done the same for decades) for the equipment they sell with 3 year and 5 year warranties. Like Dell, IBM, HP/Compaq, Asus, you name it.
There's a 275W PSU in my Dell Optiplex GX620, and it's running a 2.8Ghz Pentium-D, 4GB of ram, a 7200RPM drive and an x600. It's been running on my office desk for two years, and I can 100% guarantee that a good six months of that was at full load. The Optiplex 745 right next to it has the same exact PSU, but with a 3.4Ghz Pentium-D, 4GB of ram, a 7200 RPM drive and a DVDRW burner. For it's year of life on my desk, it has probably seen six months worth of 100% usage too.
And these are power supplies that are like 2" across, by about 2" deep, by about 7" long: slimline platform. And I don't ever expect any problems out of these boxes, because our entire company has probably more than a thousand of these units with zero PSU issues.
So in other words, research would be your friend right about now.